The usefulness of an RF (radio frequency) or microwave electromagnetic field for the purpose of linear position measurement is recognized in Prior Art Such devices can operate with either an RF or microwave excitation. When an electromagnetic field is excited near a movable object, the parameters of the electromagnetic field, such as resonant frequency, phase or amplitude, vary with the change of position of the movable object. The electromagnetic field parameters may be converted into an electronic indication of position, displacement, velocity, or acceleration of the movable object. In particular, the state of the art is shown in U.S. Pat. No. 6,819,208 “Electromagnetic linear actuator with position sensor,” 2004, in which Peghaire, et al, disclose a ferromagnetic actuator with a ferromagnetic circuit defining an axial travel interval of a ferromagnetic armature for axially driving a rod between two extreme positions in which the armature bears against poles of the ferromagnetic circuit. Resilient return means is provided to hold the valve at rest in a middle position between the extreme positions, and at least one coil is carried by the circuit, enabling the armature to be brought alternately into each of the two extreme positions. The rod carries a radially-magnetized bar of a length not less than the travel distance of the armature, and the housing carries at least one magnetic flux sensor placed in a zone having low exposure to the field created by the coil(s).
Prior Art also recognizes the usefulness of applying resonant sensing elements for measuring physical parameters, such as identifying the type of material, its moisture content, etc. When a material to be measured is placed within an electromagnetic field that is excited in a resonator being fed by an RF or microwave generator, which sweeps through a range of frequencies, the resonant frequency of the resonator shifts in relation to the properties of the material. This shift in resonant frequency can be measured or compared with another frequency, e.g. with the resonant frequency of the same resonator in another mode; see U.S. Pat. No. 3,458,808 issued to Nils Bertil Agdur on Jul. 29, 1969. In this patent, an apparatus for measuring a property of a material comprises, at least: one high frequency sweep-oscillator having a frequency periodically varying in a given range of frequencies, a cavity resonator having two resonant frequency peaks, and indicator means connected to the cavity resonator for producing a signal dependent on the difference of time of occurrence between the two frequency peaks.
When an electromagnetic field is disposed within a volume, it is known that placing a dielectric, conductive, or magnetic material within the volume can alter the parameters of the field. For example, the wave velocity of the electromagnetic field may change. A change in wave velocity leads to a change in phase delay or a change in resonant frequency. Such a change can be measured and utilized to indicate a parameter of the material. The respective influences from a dielectric, conductive, or magnetic material differ, and depend on the distribution of the electric and magnetic fields within a measured volume, see V. A. Viktorov, B. V. Lunkin and A. S. Sovlukov, “Radio-Wave measurements” [in Russian], Moscow: Energoatomizdat, 1989, pp. 148–153.
The application of slow-wave structures according to Prior Art for measuring liquid level and angular position (see patents: U.S. Pat. No. 6,293,142 B1 and U.S. Pat. No. 6,393,912 B2, both by Yu. N. Pchelnikov and D. S. Nyce) teaches the significant decrease of physical dimensions and resonant frequency of a sensing element. In these patents a sensing element, fabricated as a section of a slow-wave structure (SWS), is connected to a measuring circuit comprising an RF oscillator and a converter which converts the resonant frequency of the sensing element SWS into a level reading, in the first example, or to an angular position reading, in the second example.
The use of a SWS sensing element enables the control of electric and magnetic field distribution in the transverse and in the longitudinal directions. The use of coupled slow-wave structures makes it possible to split the electric and magnetic fields in the transverse direction (see Yu. N. Pchelnikov, “Features of Slow Waves and Potentials for Their Nontraditional Application,” Journal of Communications Technology and Electronics, Vol. 48, #4, 2003, pp. 450–462). Splitting of the electric and magnetic fields can provide additional slowing of the electromagnetic wave. Splitting them in the transverse direction can also enhance the dependence of the electromagnetic field parameters on the distance between the slow-wave structure and a conductive target.
Slowed electromagnetic waves and slow-wave structures are also well known in the field of microwave engineering, see J. R. Pierce, “Traveling-Wave Tubes” D. Van Nostrand Company, Inc., Princeton, N.J., 1950, Dean A. Watkins “Topics in Electromagnetic Theory”, John Wiley & Sons, Inc., and “Radio-Wave Elements of Engineering Devices Based on Slow-wave Structures,” [in Russian] Moscow: Radio and Communications, 2002).
Slow waves are electromagnetic waves propagating in one direction with a phase velocity νp that is smaller than the velocity of light, c, in vacuum. The ratio c/νp is called the deceleration factor, or slowing factor. It is designated as N. In most practical applications, slowed electromagnetic waves are formed in slow-wave structures by coiling one or two conductors, for example, into a helix, or radial spiral (Prior Art), which geometrically increases the path length traveled by the wave. Such a curled conductor is called an “impedance conductor”. It is commonly paired with another conductor that is not curled, called a “screen conductor”.
Additional deceleration, in addition to the geometric path length, can also be obtained due to positive electric and magnetic coupling in a coupled slow-wave structure. In this case, both conductors are coiled, and have the configuration of mirror images flipped by 180° relative to a plane of symmetry, see Yu. N. Pchelnikov, “Comparative Evaluation of the Attenuation in Microwave Elements Based on a Spiral Slow-Wave System”, Soviet Journal of Communication Technology and Electronics, Vol. 32, #11, 1987, pp. 74–78.
Slow-wave structure-based sensitive elements are known in the art, see V. V. Annenkov, Yu. N. Pchelnikov “Sensitive Elements Based on Slow-Wave Structures” Measurement Techniques, Vol. 38, #12, 1995, pp. 1369–1375. Slowing of an electromagnetic wave leads to a reduction in the dimensions of a sensing element for a given resonant frequency. Thus, by using the advantages of electrodynamic structures, a relatively small sensing element can operate at relatively low frequencies. A lower operating frequency is more convenient to generate, and more convenient for the conversion circuit which produces a desired output signal. An operating frequency can be chosen so that it is low enough to provide the above advantages, but still high enough to provide high accuracy and a high speed of response.
The low electromagnetic losses at relatively low frequencies (a few megahertz (MHz) to tens of MHz) also helps to increase the accuracy and sensitivity of the measurement. In addition, slowing of the electromagnetic wave leads to concentration of the energy in both the transverse and longitudinal directions. This results in an increase in sensitivity, proportional to the slowing factor N, see Yu. N. Pchelnikov, “Nontraditional Application of Surface Electromagnetic Waves” Abstract Book, First World Congress on Microwave Processing, 1997, pp. 152–153.
In both the Prior Art and in the present invention, one or more parameters of an electromagnetic field are measured. Some of the Prior Art methods and the present invention use one or two resonators, placed near a movable object of which the position is to be measured. Changes in the position of the movable object result in changes of the electromagnetic parameters of the resonator(s). The resonators are connected to a measuring circuit comprising an RF or microwave signal generator, which is used to excite an electromagnetic field.
Devices according to the Prior Art exhibit several problems that can be overcome by the present invention. Previous methods have low accuracy, sensitivity, and resolution at relatively low frequency, increasing only with a substantial increase in the operating frequency. However, an increase in frequency is accompanied by an increase in electromagnetic losses, such losses limiting the accuracy of the measurement. It is also generally known that a higher operating frequency can increase the cost of the associated electronic circuitry. The previous methods therefore require complex and expensive equipment. Thus, there is a need in the art for an electromagnetic method and apparatus for monitoring position that has greater sensitivity, resolution, and lower cost.